The present invention relates to a technique for irradiating an object surface by a light source and recognizing an identification mark from the reflection therefrom. More specifically, the invention relates to a technique which raises the identification mark recognition accuracy by optimizing parameters of the light source so as to generate a good contrast image.
Magnetic disk drives comprise a head gimbal assembly (hereinafter, denoted as an HGA) which has a head/slider attached to a suspension assembly. A serial number composed of a combination of letters, numbers and the like is formed on the head/slider. This serial number is used to identify the slider wafer or the HGA, facilitating production management and failure analysis. To read a serial number marked on a surface of a head/slider, it is necessary to supply a good contrast image to a pattern recognition device.
Identifiers such as serial numbers can be recognized using grayscale images which have only brightness information. Therefore, a light source such as a halogen lamp or an LED irradiates the head/slider to generate a grayscale image of the serial number with a background from the reflection of the light incident on the head/slider. The generated grayscale image is sent to a pattern recognition device. In the pattern recognition device, the serial number in the grayscale image is compared with preliminarily prepared standard patterns for recognition by template matching using the normalized correlation method.
In the normalized correlation method, similarity between input image f(x) and standard image g(x) is scored according to Equation 1 where fa is the average of the input image and ga is the average of the standard image. The grayscale image is characterized by absolute density values and contrasts. Since the density value of each pixel is normalized by the average of all pixels in this normalized correlation method, it is not necessary to consider the absolute density values of the input image for raising recognition ratio. In raising the recognition ratio, attention can be focused on contrasts only. Symbol a in Equation 1 corresponds to the distance by which the standard pattern is shifted for template matching.